function  hidden_surface_mesh ()
% Copyright 2000, J.E. Akin.  All rights reserved.
%  ------------------------------------------------------
%  3-D hidden plot of a membrane or shell mesh geometry
%  ------------------------------------------------------
 %? global az el   ? these are not changing with new view

% c_x    = x coordinates of nod_per_el line polygon
% c_y    = y coordinates of nod_per_el line polygon
% msh_typ_nodes   = connectivity list for elements, nt x nod_per_el
% loop   = corners for nod_per_el line polygon
% nod_per_el = Nodes per element
% np     = Number of Points
% nt     = Number of elements
% pre_e  = Element items before connectivity list
  pre_e  = 1 
  post_e = 0
% pre_p  = Nodal items before coordinates
  pre_p = 1
  post_p = 1
% msh_bc_xyz = Nodal coordinates (with preceeding data)
% t_x    = x coordinates of nod_per_el corners
% t_y    = y coordinates of nod_per_el corners

  %b if ( nargin == 0 )
  %b end % if no arguments

%    Read coordinate file and connectivity file
%    integer bc code, real xy pairs for np points (pre_p = 1)
  load msh_bc_xyz.tmp ;  

%    Set control data: number of points
  np = size (msh_bc_xyz,1) ;   % number of nodal points  
  fprintf ('Read %g mesh coordinate triplets \n', np)
  ns = size (msh_bc_xyz,2) - pre_p ;   % space dimension
  ns = ns - post_p 
  if ( ns < 3 )
    error ('This is not a 3D mesh, use mesh_plot')
  end % if not 3D data
  disp(msh_bc_xyz(1:2,pre_p+1:pre_p+3))
  
%    Set control data: number elements
  load  msh_typ_nodes.tmp ; % nod_per_el nodes per element
  nt = size (msh_typ_nodes,1) ;          % number of elements in mesh 
  nod_per_el = size (msh_typ_nodes,2) - pre_e ; %b-1 ; % nodes per elem
  %b nod_per_el = nod_per_el - post_e ;
  fprintf ('Read %g elements connections \n', nt)
  fprintf ('each with %g nodes \n', nod_per_el)
  disp(msh_typ_nodes(nt, pre_e+1:pre_e+nod_per_el))

  x (np)      = 0.       ;       % pre-allocate array x
  y (np)      = 0.       ;       % pre-allocate array y
  z (np)      = 0.       ;       % pre-allocate array z
  t_nodes (nod_per_el) = 0 ;    % Optional pre-allocation
  t_x (nod_per_el) = 0 ;        % Optional pre-allocation
  t_y (nod_per_el) = 0 ;        % Optional pre-allocation
  c_x (nod_per_el + 1) = 0 ;    % Optional pre-allocation
  c_y (nod_per_el + 1) = 0 ;    % Optional pre-allocation
  loop (nod_per_el + 1) = 0 ;   % Optional pre-allocation

%                set constants
  [loop] = get_El_Loop (nod_per_el) ;

  % msh_bc_xyz has: pre_p items then: x, y
  x = msh_bc_xyz (1:np, (pre_p+1)) ; % extract x column
  y = msh_bc_xyz (1:np, (pre_p+2)) ; % extract y column 
  z = msh_bc_xyz (1:np, (pre_p+3)) ; % extract y column 

%    Initialize plots
   xmax = max (x)      ; xmin = min (x) ;
   ymax = max (y)      ; ymin = min (y) ;
   zmax = max (z)      ; zmin = min (z) ;
   clf                               % clear graphics
   axis ([xmin, xmax, ymin, ymax, zmin, zmax])   % set axes
   axis('equal')
% view([52.5,30])


   hold on                           % hold image for plots
   xlabel ('X')                      % add label 
   ylabel ('Y')                      % add label
   zlabel ('Z')                      % add label
   title (['FEA Surface Mesh Geometry : ', ...
           int2str(nt),' Elements, ', int2str(np), ...   
           ' Nodes, (', int2str(nod_per_el), ' per Element)'])

%    Loop over all elements
  for it = 1:nt  ;

%      Extract corner connectivity 
    %b t_nodes = msh_typ_nodes (it, (pre_e+2):(nod_per_el+pre_e+1)); 
    t_nodes = msh_typ_nodes (it, (pre_e+1):(pre_e+nod_per_el)); 
    if ( all ( t_nodes > 0 ) ) %B

%      Extract corner coordinates
      t_x  = x (t_nodes) ;         % x at those nodes, only
      t_y  = y (t_nodes) ;         % y at those nodes, only
      t_z  = z (t_nodes) ;         % z at those nodes, only

%      Plot this polygon
      c_x  = t_x (loop) ;       % x for nod_per_el line polygon
      c_y  = t_y (loop) ;       % y for nod_per_el line polygon
      c_z  = t_z (loop) ;
      fill3 (c_x, c_y, c_z, 'w'  )           % plot nod_per_el lines
    end % if all
  end % for over all elements
  grid
 

% end % if show labels
%       -depsc -tiff            % for an eps version
% print ('-dpsc', ['hidden_surface_mesh'])                     
  hold off
%  v_text = ['Created hidden_surface_mesh.ps']) ;
% fprintf (1,'%s', v_text) ; fprintf (1, ' \n' )
% end of hidden_surface_mesh